This invention relates to ground velocity detectors in general and more particularly to a ground velocity detector using the natural radiation from terrain.
It is important in aircraft navigation to determine the velocity of the aircraft with respect to the ground. Prior art devices for determining ground velocity include airborne Doppler navigation radar. However, this system involves the transmission of signals by the aircraft. Such transmissions make a military aircraft vulnerable to detection and destruction by enemy defensive systems. Similarly, the prior art includes velocity sensors which use the pattern of back-scattered power from a radiating antenna to determine ground velocity, as disclosed in U.S. Pat. Nos. 3,838,424 and 3,974,500. Like the Doppler system, this system also is vulnerable to defensive countermeasures because of its use of a transmitting antenna.
Prior art systems also include a passive sensor system which used antennas sited in the front and rear of an aircraft to sense natural terrain radiation. The relative time delay in the sensed signals measured the ratio of ground velocity to altitude. However, another method was needed to determine the aircraft's altitude. The use of active radar to measure altitude would make this system as vulnerable to countermeasures as the Doppler system.